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I was motivated by the -5 deg F temperatures and snow today to finally test the rear compartment heating (cabin heat, not seat heaters). In the front, with the temperature set to 65 deg, after a minute I'm getting toasty hot air from the vents. But in the rear, with the temperature set to 79 degrees and fan on 7 or 8, after 5 minutes I'm getting cool to mildly warm air through the 2nd and 3rd row air vents.
So before I schedule a service appointment, I thought I'd ask if others are seeing the same heater performance and is this normal for the MX?
Thanks.

Are there passengers in the rear seats, or just testing the air flow. When the seats are not occupied the rear is neither air conditioned or heated. So perform the test with a passenger in the seat (or heavy object to activate the seat belt warning). See page 118 of the Owner's Manual.

Note: When set to AUTO, the rear heating and air conditioning turn off when Model X detects that
there are no occupants in the second or third row seats. To override this energy saving feature,
touch the AUTO button located above the Climate On/Climate Off setting. When doing so, the
setting is saved until you manually change it by touching AUTO a second time.

Are there passengers in the rear seats, or just testing the air flow. When the seats are not occupied the rear is neither air conditioned or heated. So perform the test with a passenger in the seat (or heavy object to activate the seat belt warning). See page 118 of the Owner's Manual.

Note: When set to AUTO, the rear heating and air conditioning turn off when Model X detects that
there are no occupants in the second or third row seats. To override this energy saving feature,
touch the AUTO button located above the Climate On/Climate Off setting. When doing so, the
setting is saved until you manually change it by touching AUTO a second time.

Click to expand...

Aljohn, thanks for the reply. Yes, I was aware of this but forgot to mention in my post that I was in manual mode ("Auto" turned off) for all 3 categories of rear HVAC (HVAC on/off, temp, fan). Just to be complete, I also tested it with an adult in a 2nd row seat. My local SC is looking into it right now to see what they can determine remotely.

Aljohn, thanks for the reply. Yes, I was aware of this but forgot to mention in my post that I was in manual mode ("Auto" turned off) for all 3 categories of rear HVAC (HVAC on/off, temp, fan). Just to be complete, I also tested it with an adult in a 2nd row seat. My local SC is looking into it right now to see what they can determine remotely.

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I see. Thought I would make the suggestion. My X is at the Service with my 6-month punch list, so I can't test the heating system right now. Good luck on trouble shooting. Always, you can call the 800 # and they are always helpful, and have various models at their disposal. Best using a cell phone while in the vehicle.

I kinda tested this last night in the cold. Overrode auto on the rear climate and forced it on, HI setting, and turned fan up also. It blew ice cold air. No one was back there, but it was overridden so should have put out heat still IMHO. I did go then sit in a back seat, and it took a while but did start to warm up it seemed, only after I was actually sitting in the seat.

Can someone test that as well and confirm? Override with no weight detected blows cold still, but if actual person in back gets warm?

I kinda tested this last night in the cold. Overrode auto on the rear climate and forced it on, HI setting, and turned fan up also. It blew ice cold air. No one was back there, but it was overridden so should have put out heat still IMHO. I did go then sit in a back seat, and it took a while but did start to warm up it seemed, only after I was actually sitting in the seat.

Can someone test that as well and confirm? Override with no weight detected blows cold still, but if actual person in back gets warm?

Click to expand...

@travwill , I had no difference with the seat empty or occupied. But, in both cases I saw what you did: with time, the air warmed slightly, but only from cold to cool. I put a temp probe 2 inches into front and rear vents, set both front and rear to 75 deg, fan=4, manual, outside air. After 5 min, front air temp was 128 deg F, the rear was 88 deg. The temps stabilized at these temps. But that 88 deg inside the duct, felt cool at the outlet. The front air felt warm to hot, just like every other car heater I've had.

I do not have an X, but guessing based upon my experience with rear seat heating in other cars -- for full warmth the system probably has to heat up the ducts themselves. Depending upon the length of the duct and the material (plastic seems likely), it may take some time to heat the duct to the point where it is not cooling the air flowing through it. Another factor may well be conductive heat losses through the duct to the floor or other outside surface. The amount of heat loss from the ductwork would of course depend upon what insulation there is, if any. Space limitations may have prevented use of much insulation.

@travwill , I had no difference with the seat empty or occupied. But, in both cases I saw what you did: with time, the air warmed slightly, but only from cold to cool. I put a temp probe 2 inches into front and rear vents, set both front and rear to 75 deg, fan=4, manual, outside air. After 5 min, front air temp was 128 deg F, the rear was 88 deg. The temps stabilized at these temps. But that 88 deg inside the duct, felt cool at the outlet. The front air felt warm to hot, just like every other car heater I've had.

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Thanks for testing. Going to again as well as it may have just been the recirculating warmer air due to front heating up starting to come out rear vents finally. It shouldn't just blow ice cold air when on high for more than 20 seconds for so, and it did for 2 minutes plus. Going to ask SC also about the issue, thanks.

I discovered the same issue tonight. Cold air blowing on my kids on the drive home. 38 outside and front and rear set at 72 degrees. I had to manually set fan to the lowest setting to stop the cold air. Something is wrong when your rear vents blow cold instead of hot.

I discovered the same issue tonight. Cold air blowing on my kids on the drive home. 38 outside and front and rear set at 72 degrees. I had to manually set fan to the lowest setting to stop the cold air. Something is wrong when your rear vents blow cold instead of hot.

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I ran a longer test during a drive over over 20 minutes with the rear climate control in Manual-75 deg-fan 4, and the air never got warm. I'm booked for an SC visit on this issue in Jan and will post what they find.
I'm wondering if others, like me, have had this problem for a while but haven't discovered it yet because unless you intentionally test for it, or have a rear passenger complain, you would never know there was an issue.

I too have the same issue with rear heat. I think it may just be the size of the cabin and how much air the system has to warm in relation to losses when the MX is moving with the temps dropping below 0. Not to mention the cold feet issue in the front cabin, which I found to be minimized if I turn on the rear cabin manually even if no one is back there.

I discovered the same issue tonight. Cold air blowing on my kids on the drive home. 38 outside and front and rear set at 72 degrees. I had to manually set fan to the lowest setting to stop the cold air. Something is wrong when your rear vents blow cold instead of hot.

Click to expand...

Better to turn off the malfunctioning rear HVAC altogether and heat the cabin with the front system; the vents at the rear of center console are part of the front system so it is adequate for middle seat passengers as long as cold air is not blowing on them from the pillar vents. The 5-seat model does not have a rear HVAC since they deem the front adequate for those seats.

I tried the seat ventilation (cool) on max and decided that the I'd pass on that option, as the only ventilation was at the very front portion of the seat and never achieved an even cool temp, in the low flow rate during my demo test.

OTOH,The only way to get, AFAIK, any heat to the rear seats, in a 5 seat configuration, is to pay for the sub zero package, which provides seat heaters to those rear seats.

Can someone say whether or not the model X has the vent outlets under the two front seats, as in the Model X? If so, they send heated (or cooled) air to the rear seating area when the air flow is set to the floor position (arrow facing down on the control screen). Not necessarily adequate, but it is likely part of the front heating even if rear heating is not on. But Model X may differ from Model S.

Can someone say whether or not the model X has the vent outlets under the two front seats, as in the Model X? If so, they send heated (or cooled) air to the rear seating area when the air flow is set to the floor position (arrow facing down on the control screen). Not necessarily adequate, but it is likely part of the front heating even if rear heating is not on. But Model X may differ from Model S.

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Will try to sum it up. All Xs have vents under the front seats, that blow forward to heat the cabin (not rear). The middle console vents though do blow backwards.

The 5 seater depends on all of this only, mainly heat from center console going back.

The 6/7 seater do not have any vents in 2nd/3rd row on floor, just two in the side pillar in 2nd row, and two in side pillar in 3rd row. The A/C works pretty well from these vents I recall in hot Summer. The heat though doesn't. It does take a long while to warm the air up, and it is never as warm as the front air. But, in really cold weather, after quite a while, it does reach a temp that would warm up the cabin or help as its warmer than outside for sure. Maybe it gets up into the 70s max I've felt.

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